Coated articles are known in the art for use in window applications such as insulating glass (IG) window units, vehicle windows, and/or the like. It is known that in certain instances, it is desirable to heat treat (e.g., thermally temper, heat bend and/or heat strengthen) such coated articles for purposes' of tempering, bending, or the like in certain example instances. Heat treatment of coated articles typically requires use of temperature(s) of at least 580 degrees C., more preferably of at least about 600 degrees C. and still more preferably of at least 620 degrees C. Such high temperatures (e.g., for 5-10 minutes or more) often cause coatings to break down and/or deteriorate or change in an unpredictable manner. Thus, it is desirable for coatings to be able to withstand such heat treatments (e.g., thermal tempering), if desired, in a predictable manner that does not significantly damage the coating.
In certain situations, designers of coated articles strive for a combination of desirable visible transmission, desirable color, low emissivity (or emittance), and low sheet resistance (Rs). Low-emissivity (low-E) and low sheet resistance characteristics permit such coated articles to block significant amounts of IR radiation so as to reduce for example undesirable heating of vehicle or building interiors. Often, more IR radiation being blocked (including reflected) is accompanied by less visible transmission.
U.S. Pat. No. 7,597,965 discloses a low-E coating with an NiCr absorber layer in the lower dielectric stack. However, the example coating in the '965 patent is designed for a high visible transmission, and indeed has a visible transmission (Tvis or TY) of 59%. Lower visible transmissions are often desirable. For example, it is often desirable for aesthetic and/or optical purposes to provide coated articles (including low-E coatings) having visible transmissions of no greater than 50%, more preferably no greater than 40%, and sometimes no greater than 35%. However, when visible transmission of a coated article is reduced via a low-E coating design, the film side reflectance of the coating typically increases.
U.S. Pat. No. 7,648,769 discloses a low-E coating with an NiCr absorber layer provided in the middle dielectric stack, but not in the upper and lower dielectric stacks of the coating (e.g., see FIG. 1 of the '769 patent). Example 1 in the '769 patent realizes, measured monolithically, a visible transmission of 54.5% and a film side reflectance of 19.5%, and when measured in an insulating glass (IG) window unit the values change to a visible transmission of 50% and a film side reflectance of 23%. In a similar manner, Example 2 in the '769 patent realizes, measured monolithically, a visible transmission of 67.5% and a film side reflectance of 11.5%, and when measured in an insulating glass (IG) window unit the values change to a visible transmission of 62% and a film side reflectance of 17%. It will be appreciated that the examples in the '769 patent do not realize a simultaneous combination of both (i) low visible transmission, and (ii) low film side reflectance. Instead, the examples in the '769 patent teach that when visible transmission goes down, film side reflectance goes up.
It will also be explained herein, in the detailed description section, that providing an absorber layer only in the middle dielectric stack of a low-E coating having a visible transmission of about 40% results in an undesirably high visible film side reflectance (RfY) of over 30%.
Thus, it will be appreciated that it has been difficult to achieve coated articles, including low-E coatings, having a combination of both (i) desirably low visible transmission, and (ii) low film side reflectance. It will be apparent to those skilled in the art that there exists a need in the art for a coated article having low emissivity (or low sheet resistance) and a combination of both low visible transmission (e.g., no greater than 50%, more preferably no greater than about 40%, and most preferably no greater than about 35%) and low film side reflectance.